The mental fight against cancer can surely cause the anger. As well as the pain meds, and steroids as others have mentioned. My husb had some serious personality changes over the course of treatment. It was so hard to not take personally, and the docs did give him some anti anxiety meds to help him a bit.
Definitely communicate the feelings to the docs as I'm sure your brother is just focused on getting rid of the cancer and not so much about his moods.